This invention concerns a method of making a spirally-wound tube from a hollow thermoplastic profile having a substantially rectangular cross-section, which method comprises the steps of: winding the thermoplastic profile around a drum, joining adjacent loops together by welding, smoothing the outer weld joint by means of extra heat and a smoothing body resting against the weld joint, and feeding the tube axially forward by means of freely rotating, radial press rolls influencing the thermoplastic profile during the first winding around the drum.
The invention also concerns an apparatus for carrying out the above-described method, wherein the apparatus comprises: a cylindrical drum around which a thermoplastic profile is to be wound; radially oriented, freely rotating press rolls arranged along a periphery of the drum at an incoming end thereof; welding means for joining adjacent loops of the thermoplastic profile together; and heating and smoothing means for a weld joint formed on the outside of the tube.
Until now, this method has been carried out by means of a drum rotating along its entire length around which the thermoplastic profile is wound and on which the welding is performed. The weld joint on the outer surface of the tube is smoothed when the tube lies against the drum. This is done by supplying extra heat and allowing a stationary smoothing body to lie against and slide along the heated weld joint thereafter, during the rotary motion of the tube. The weld joint inside the tube has been formed against the rotating drum. A similar method is known, for example, from Patent Specification FI95219.
In order to make sure that the material in the weld joint is hot/molten enough and that there is no risk of a so-called cold joint, the welding burr on both the outer and the inner surface of the tube has to be of optimum size. A customary welding drum makes it somewhat more difficult to control the size of the inner welding burr, and at the same time, there is a small risk that the surface of the inner weld joint is affected by the axial sliding of the tube along the drum as long as the plastic material is molten.